1. Field of the Invention
The present invention relates to the technical field of signal modulation and, more particularly, to a Delta-Sigma modulator.
2. Description of Related Art
Typically, Delta-Sigma modulation is used for converting the analog signal to the digital signal via comparing a reference voltage formed by accumulating a fixed voltage with an input voltage. When the analog signal is in the states that the difference of the analog signal is instantaneously too large, the transmitting speed of the analog signal is too fast, or the slope of the analog signal is too large, the reference voltage cannot easily trace the input analog signal, which results in that the digital signal processed by the Delta-Sigma modulation is distorted and the input analog signal cannot be responded in real time.
FIG. 1 shows the diagram according to the prior Delta-Sigma modulation, which includes an integrator 11 and a quantizer 12. The prior Delta-Sigma modulation further utilizes a feedback means to generates digital output signal. FIG. 2 shows a comparison for the analog input signal and the digital output signal of the prior Delta-Sigma modulation. When the change of the analog input signal is fast, the density of the digital output signal is tight. Conversely, when the change of the analog input signal is slow, the density of the digital output signal is sparse. The prior Delta-Sigma modulation uses the sampling means to convert the signal, so that there is a functional relation between the input signal, the output signal and the sampling rate. Therefore, when the frequency of the input signal is too fast and the sampling rate is fixed, the output signal will be distorted.
The aforementioned Delta-Sigma modulation techniques are used as an analog/digital converter (ADC) for converting the analog signal to the digital signal. The digital signal can be restored to the analog signal via a low pass filter. In addition, the prior Delta-Sigma modulation cannot directly scale up or scale down the input signal in the modulation. One solution for scaling up/down the input signal is to use an additional multiplier to scale up/down the input signal at certain processing point such as before the input signal modulation or after the output signal is processed. Another solution is to use at least one adder to scale up the output signal. However, the additional multiplier and adder both occupy much space, which makes chip miniaturization difficult. Further solution is to utilize a RC circuit to control the signal level after low pass filtering, but the RC circuit is complex and thus the cost is increased.
Therefore, a need exists for providing a Delta-Sigma modulator so as to be capable of controlling signal level without RC circuits or additional multipliers and eliminating the distortion that is caused by the sampling rate.